The invention relates to an electromechanical actuating drive for a valve, in particular of a turbine, with a push rod for setting an opening position of the valve and with an electric motor for driving the push rod. The invention relates, furthermore, to a steam turbine with a valve and with an associated actuating drive.
A turbine, in particular a steam turbine, usually has a considerable number of valves which may be used, for example, as fresh-steam, intercepting or bypass valves and also as scram valves. In such cases, the opening position of each of these valves serves for setting a respective material or steam stream and is capable of being set, for example, via a push rod assigned to the respective valve. Such a push rod is an integral part of an actuating drive assigned to the valve. In this context, the actuating drive may have to satisfy stringent requirements, particularly as regards actuating force and actuating speed. For the valve to have a high degree of reliability, it may be necessary, for example, that the actuating drive have an actuating force of about 200 kN and an actuating time of about 100 ms.
EP 0 040 732 A1 specifies an actuating drive for steam turbine valves which has a decentral hydraulic system. The hydraulic system of such an actuating drive is integrated to form a compact drive block arranged on the valve housing, so that only a cable system is still necessary for supplying energy to the actuating drive. If oil is used as hydraulic fluid, this may lead, under extremely unfavorable circumstances, to an ignition of the oil and therefore to a fire in the steam turbine.
DE 44 46 605 A1 specifies a valve for a steam turbine which has a valve spindle with a valve cone arranged thereon. The valve spindle is driven via an electric motor which is connected to the valve spindle via an electromagnetically actuated coupling. For automatic selfclosing of the valve, the latter comprises a cup spring system. The electromagnetic coupling is connected to a threaded bush which cooperates with the valve spindle guided fixedly in terms of rotation and which thus moves the latter axially. The threaded bush is designed as a spherical threaded bush, so that it acts upon the valve spindle with low play and low friction. Since the valve spindle is guided fixedly in terms of rotation, that is to say the latter can be moved up and down only axially when the threaded bush rotates, there is no need for a drive having a translational effect. Instead, an electric motor rotating in two different directions is sufficient for this purpose. However, the electric motor must have a torque safety device, in order thereby to prevent the situation where, for example during the closing of the valve, the valve cone adjoining the valve spindle at one end does not, when coming to bear against the sealing seat assigned to it, suffers damage or damages said sealing seat.
DE 36 18 479 A1 discloses a fitting for shutting off a flow, with a housing, in which a closing piece is movable between the open and shut end positions with the aid of a thread, with a rotary drive (drive motor) which is switched off with a time delay. A gear with two concentrically arranged ballscrew spindles is arranged between the rotary drive and the closing piece. A purely mechanical setting of a torque is achieved by means of this double-spindle system, the setting being carried out at a correspondingly high rotational speed without much effort.
An object on which the invention is based is, therefore, to specify an actuating drive for a valve, in particular of a turbine, in which, along with a particularly low risk of fire, fault-free operation of the valve is ensured. A further object of the invention is to specify a steam turbine with a low risk of operation being disrupted by a fire.
This and other objects are achieved by means of an electromechanical actuating drive with a push rod for setting an opening position of the valve, and with an electric motor for driving the push rod, the push rod and the electric motor being connected to one another via a transmission device, by means of which a changing torque can be generated, depending on the axial displacement of the push rod.
An electromechanical actuating drive with a transmission device, by means of which a changing torque can be generated, depending on the stroke of the push rod, is particularly advantageous in those valves in which the force to be exerted on the push rod during the closing or the opening of the valve changes as a function of the stroke of the push rod. This may occur in valves, for example in a steam turbine, in which a prevailing steam pressure is present on the valve cone and supports the latter during the closing operation or keeps the valve cone closed. In such a valve, initially a high force (steam pressurexc3x97effective valve cone surface) has to be overcome in order to open the valves to some extent. As soon as a particular opening cross section is afforded, pressure relief takes place, so that, during the further opening operation, in each case a lower force is necessary in order to displace the push rod. The transmission device may be designed appropriately, depending on the size, load and requirement of the valve, in particular the opening and closing operation (closing speed, etc.). Moreover, using an electric motor markedly reduces a risk of fire, as compared with actuating drives with oil hydraulics.
The electric motor is preferably designed as a variable-speed synchronous motor. A synchronous motor of this type makes it possible, particularly in conjunction with a power amplifier, to have highly accurate position regulation with a control accuracy of about 0.1 mm, along with a motor power up to about 30 kW. The power amplifier acts, in this case, as a frequency converter and allows a general or position-dependent reduction in the motor current and in the motor rotational speed. At the same time, all the drive powers and drive actuating speeds which are required in practice can be afforded by an appropriate adaption of the operating parameters.
In order to convert the rotational movement of the electric motor into a linear movement provided for driving the push rod, the transmission device is designed preferably as a crank mechanism, a planetary gear or the like.
Feed-dependent torque/force transmission is possible by means of a crank mechanism. Moreover, in the case of a quick closing of the valve, such as occurs, for example, during a quick-action closure of a valve in a turbine, necessary limit damping may already be achieved by means of the crank mechanism. Additional devices for speed reduction during the last stroke of the valve cone (sealing piece) into the valve seat may therefore be dispensed with, as required. In this case, a crank mechanism has a crank element, in particular a crank disk, which rotates about an axis of rotation and on which the push rod acts at a connection point, the connection point being at a distance from the axis of rotation of the crank element (crank disk). During a rotational movement of the crank disk, the push rod connected to it executes a linear movement along the main axis of the push rod between two maximum end positions. A very high torque can be generated in these end positions via the crank disk, since, during a rotation of the crank disk, only a very short linear distance is covered in the direction of the push axis in the respective end position.
The actuating drive is preferably designed in such a way that, when a closing position of the valve is reached, an angle in the range of between 5xc2x0 and 20xc2x0, preferably about 10xc2x0, is formed between the main axis of the push rod and a connecting line between the connection point, at which the push rod acts on the crank disk, and the axis of rotation. Thus, when the valve is in the closing position, the connection point is located in the vicinity of a maximum end position, so that, during a rotation of the crank disk, a high torque and therefore a high force can be transmitted to the push rod. Thus, it becomes possible for the valve to be opened particularly easily by means of the actuating drive, without a particularly high-power and/or variable-torque electric motor having to be provided. The push rod is connected preferably to a connecting rod which acts at the connection point, in particular a crank pin connected fixedly to the crank disk. The connecting rod is connected rotationally movably to the push rod and to the crank pin in a plane in which the main axis of the push rod lies.
By means of the transmission device, in particular the crank mechanism, feed-dependent torque/force conversion can be implemented in a simple way, even in applications where a high feed force of, for example, approximately 200 kN is necessary in order to open a valve. By a variation in the crank radius (in the distance between the connection point and the axis of rotation) or by a displacement of the operating point, that is to say the position which the connection point assumes when the valve is in the closing position, the characteristic curve (torque characteristic curve) of the crank mechanism can be adapted to different requirements. For example, for a scram valve of a steam turbine, the choice of the geometry of the crank disk may be made with a view to providing a very high force for opening the valve and to the return force, acting in the event of a quick-action closure, of a storage spring assembly, in such a way that as high an acceleration force as possible is exerted on the crank mechanism.
An actuating drive with such a transmission device also affords the advantage that limit damping can be assumed even by the rotating crank disk. This is particularly important in the event of a quick-action closure, since, on the one hand, the valve must close with an average speed of, for example, 2 m/s and, on the other hand, the valve cone should come to rest on the valve seat only at a speed of, for example, 0.07 m/s. To be precise, without such limit damping, the valve cone would move, approximately unbacked, into the valve seat and could be damaged itself or could damage the valve seat.
As a result of a storage spring assembly (return spring system) which generates a uniform closing speed of the push rod in the event of a quick-action closure, there is a greater crank angle in the region of the end position, so that the crank disk is further accelerated in this region. The greater crank angle and the acceleration on the crank disk may therefore be utilized pre-eminently for limit damping in the region of the end position.
The actuating drive preferably has an electromagnetic denture clutch which ensures the transmission of force from the electric motor to the push rod, in particular to the transmission device. The electromagnetic clutch is preferably a positive clutch, in which two clutch parts engage positively one in the other and force and torque transmission thereby takes place without any appreciable contact pressure between these two clutch parts. By contrast, in a nonpositive clutch, the two clutch parts are pressed against one another with high pressure, so that force and torque transmission takes place for the substantial part as a result of this.
What is achieved by the electromagnetic clutch is that, in the event of a quick-action closure, the masses to be accelerated are extremely low and a minimal closing time is thereby achieved. Consequently, when the clutch is opened, there is a separation of the crank mechanism from the electric motor and from a rotary system assigned to the electric motor and comprising, for example, a gear and a drive shaft to the crank mechanism.
The electromagnetic clutch preferably has at least three coil systems (exciting coils) independent of one another, and, even if one coil system fails, it is ensured that the denture clutch is opened and also is kept closed. This system of coil systems operated independent of one another, in which the clutch continues to be operational if one coil system fails, is designated as the nxe2x88x921 of n safety technique. Preferably, the clutch has three coil systems independent of one another and is therefore designed according to a 2-of-3 safety technique. In order to keep the clutch closed, the force of two coils is sufficient in this case. It is still possible to open the clutch even when one coil continues to draw the clutch together and the two remaining coils are switched for release of the clutch. The clutch preferably has a spacer element, in particular a spring, which, when the clutch is in the opened state, prevents the clutch parts from engaging one into the other. The coil systems have to act counter to this spacer element during the closing of the clutch. Preferably, at the same time, the clutch is designed in such a way that all three coil systems (all n coil systems) must function in order to close the clutch.
The denture clutch preferably has a clutch disk which is rotatable about a clutch axis and which can be brought along the clutch axis into engagement with a drive shaft connected to the electric motor. In this case, the clutch axis preferably coincides with the axis of rotation of the transmission device, that is to say of the crank mechanism. The clutch has, furthermore, a clutch hub, and, in one embodiment, the clutch hub is directly connected positively to the clutch disk and the clutch hub is spaced from the transmission device via the spacer means, in particular a spring. In another embodiment, the clutch disk is connected to the clutch hub via a spacer means, in particular a spring, and can be brought along the clutch axis into engagement with the clutch hub. In both instances, in each case only a low mass has to be moved during the opening and closing of the clutch, in order to achieve a separation of the electric motor from the transmission device.
The actuating drive preferably has a return spring system which is connected to the push rod and which exerts a force on the push rod in the direction of the closing position of the valve. Particularly in the event of a quick-action closure, closing of the valve is achieved via the return spring system by a corresponding force being exerted on the push rod.
Preferably, the denture clutch is mounted by means of a clutch bearing in such a way that at most a slight force has to be applied in order to disengage the denture clutch. Preferably, too, the clutch bearing at the same time comprises a ball joint. In a particularly preferred embodiment, the clutch disk is held by at least one holding rod mounted in the ball joint. By virtue of this embodiment of the denture clutch, the latter can be disengaged by means of even only a slight force. In particular, there is not catching of the denture clutch due to high static friction, which could prevent the denture clutch from being disengaged.
The object aimed at a steam turbine is achieved by means of a steam turbine such as has an actuating valve for setting a steam stream, in which case the actuating valve can be opened and closed via an actuating drive described in more detail above.
The advantages achieved by means of the invention are, in particular, that, by virtue of the electric motor provided for driving the push rod, it becomes possible for the actuating drive to be operated without pressure oil. Consequently, particularly in use in a steam turbine, the risk of fire is especially low. Moreover, the actuating drive can be installed in any desired installation position and, in particular, may be employed without high outlay as a replacement for an actuating drive with a decentral hydraulic supply. The electric motor can be adapted to different requirements in a particularly simple way by means of suitable activation, so that the actuating drive is especially flexible. The transmission device, in particular the crank mechanism, can likewise be adapted in a simple way to the different requirements. The components of the actuating drive, in particular the electric motor and the electromagnetic denture clutch, can be produced by simple means and are therefore particularly maintenance-friendly.